Primarily size and population. The USSR in 1940 was over 40 times the size of France and had 3.5 times as many people.

The French were willing and capable of fighting the Germans. But even with British help they could only match the Germans, man to man. Once they lost the initial drive from Sedan to the coast, they fought hard and well, but by that point the Germans had already knocked out a large portion of the army. Even the troops who escaped from Dunkirk were largely out of the war, because by the time the British could transport them and the French reorganize and re-equip them the second line of defense had been broken. Thus Germany was in control of Paris and the industrial heartland of France before the French could adjust to their early defeats. They didn't have the depth to deal with blitzkrieg. Once the frontiers were taken, they were done. And the Germans had a brilliant operational plan to take the frontiers.

In comparison the Soviets did have manpower and depth. The first month against the Soviets the Germans did far better than they did in their month against France. They captured and killed more men. They captured more territory. But the Soviets had the luxury of far more men and vastly more territory. The Soviets could (and did) lose several Frances worth of men and territory. Moscow was nearly 3 times as far from the nearest German border compared to Paris. By the time the Germans got there they had run out of gas, both figuratively and literally. They were at the end of an incredibly long supply line. Their equipment was battered and worn after five months of campaigning. And then the Soviets hit them with fresh, well trained reserves. But if the Soviet Union had only been the size of a couple of Frances they never would have gotten a chance to prepare the defenses around Moscow and being the Siberian troops in.